1. Field of the Invention
The invention relates generally to the fields of digital data communications and telecommunications, and specifically to apparatus and method for simultaneously transmitting speech and data in the same communications channel between two or more terminals, each of which terminals includes a voice and data capability, such as a telephone and data terminal. The invention is particularly applicable to Electronic Private Automatic Branch Exchanges (EPABX) having voice and data capabilities, of office automation equipment, telephone systems, engineering work stations, and intelligent terminals. Data may be used by end user terminals or for control and signalling between source and destination of the voice path.
2. Description of the Prior Art
Various PABX equipment is known in the prior art, representative of which are U.S. Pat. Nos. 3,943,297; 4,028,498 and 4,136,263. First generation EPABX equipment had stored program control, and used either electromechanical switching networks, such as Xbar or reed relay; or electronic analog networks. These systems provided greater capabilities than those of a standard telephone set by activation of special codes which were deciphered by the system.
The second generation EPABX of the prior art uses digital switching networks configured either as time-space-time (TST) or space-time-space (STS), resulted in substantial size reduction. Examples of such second generation EPABX equipment are ROLM CBX and Wescon digital PABX.
Telephone instruments are evolving toward multifunction devices providing both speech communication and data transmission requirements from, in effect, a digital data interface. Thus the telephone set effectively is evolving toward the digital data interface and the EPABX to a digital data switching network as well as the standard voice network. It is therefore necessary to transmit digitized speech and data through a digital switching network. A digital switching network able to switch digitized speech and data through the network is described in U.S. Pat. No. 4,201,891 entitled Expandable Digital Switching Network, of A. Lawrence et al and is assigned to the assignee of the present invention. Another example of a switching system for voice and data is U.S. Pat. No. 4,317,962 of J. Cox et al, also assigned to the assignee of the present invention. The present invention, which may be considered to be fourth generation EPABX technology, is capable of providing simultaneous transmission of speech and data in the same PCM channel, and is particularly advantageously utilized in combination with digital switching networks such as that of aforementioned U.S. Pat. No. 4,201,891 wherein digitized speech and data are transmitted in the same PCM channels. However, in U.S. Pat. No. 4,201,891 the digitized speech and data are transmitted sequentially, whereas in the present invention, the digitized speech and data are transmitted simultaneously, e.g. within the channel times of each frame.